feat(worktrees): démarrer une session sur la branche principale ou tout worktree

Ajoute le choix « créer un worktree OU bosser sur la branche principale » au
moment de démarrer le travail, sur deux surfaces :
- bouton « Démarrer » (claude/bash) sur chaque WorktreeCard, y compris le
  checkout principal (badge « principal ») — comblait un manque : impossible
  jusqu'ici de lancer une session sur un worktree existant depuis le dashboard ;
- toggle de mode (Worktree / Branche principale) dans le formulaire d'entête
  de RepoSection.

Option git « créer & basculer » : nouvel endpoint POST /api/v1/repos/:id/session
qui lance UNE session dans le checkout principal (repo.path), avec création ou
bascule de branche optionnelle (git switch[-c]) — refusée en 409 si l'arbre est
sale. Pas de hooks ni de pré-trust (c'est le dépôt réel de l'utilisateur).

- shared : type additif StartRepoSessionRequest.
- server : git.switchBranch + WorktreeManager.startMainSession + route.
- web : store worktrees.startMainSession, UI WorktreeCard/RepoSection, i18n fr/en.
- tests : +5 unitaires (switchBranch, startMainSession) ; acceptance-p3 étendue.

packages/web/src/components/RepoSection.vue

@@ -5,7 +5,7 @@
       <BaseBadge v-if="!repo.valid" tone="red">{{ t('repos.invalid') }}</BaseBadge>
       <span class="truncate font-mono text-xs text-zinc-500" :title="repo.path">{{ repo.path }}</span>
       <div class="ml-auto flex items-center gap-2">
-        <BaseButton size="sm" :icon="Plus" @click="creating = !creating">{{ t('worktrees.new') }}</BaseButton>
+        <BaseButton size="sm" :icon="Plus" @click="creating = !creating">{{ t('worktrees.startWork') }}</BaseButton>
         <BaseButton size="sm" :icon="Scissors" :loading="busy" @click="onPrune">{{ t('worktrees.prune') }}</BaseButton>
         <BaseButton
           size="sm"
@@ -18,23 +18,58 @@
       </div>
     </header>
 
-    <form v-if="creating" class="card-inset flex flex-wrap items-end gap-2" @submit.prevent="onCreate">
-      <label class="flex flex-1 flex-col gap-1 text-xs text-zinc-400">
-        {{ t('worktrees.branch') }}
-        <input v-model="branch" class="input font-mono" placeholder="feature/…" required />
-      </label>
-      <label class="flex flex-col gap-1 text-xs text-zinc-400">
-        {{ t('worktrees.start') }}
-        <select v-model="startSession" class="input">
-          <option :value="null">{{ t('worktrees.startNone') }}</option>
-          <option value="claude">claude</option>
-          <option value="bash">bash</option>
-        </select>
-      </label>
-      <label class="flex items-center gap-1 text-xs text-zinc-400"><input v-model="newBranch" type="checkbox" /> {{ t('worktrees.newBranch') }}</label>
-      <BaseButton type="submit" variant="primary" size="sm" :loading="busy" :disabled="branch.trim() === ''">
-        {{ t('worktrees.create') }}
-      </BaseButton>
+    <form v-if="creating" class="card-inset flex flex-col gap-2" @submit.prevent="onCreate">
+      <!-- mode : créer un worktree dédié, ou bosser directement sur le checkout principal -->
+      <div class="flex flex-wrap items-center gap-3 text-xs text-zinc-400">
+        {{ t('worktrees.modeLabel') }}
+        <label class="flex items-center gap-1.5"><input v-model="mode" type="radio" value="worktree" /> {{ t('worktrees.modeWorktree') }}</label>
+        <label class="flex items-center gap-1.5"><input v-model="mode" type="radio" value="main" /> {{ t('worktrees.modeMain') }}</label>
+      </div>
+
+      <div class="flex flex-wrap items-end gap-2">
+        <!-- mode worktree : branche + créer/existante -->
+        <template v-if="mode === 'worktree'">
+          <label class="flex flex-1 flex-col gap-1 text-xs text-zinc-400">
+            {{ t('worktrees.branch') }}
+            <input v-model="branch" class="input font-mono" placeholder="feature/…" required />
+          </label>
+          <label class="flex flex-col gap-1 text-xs text-zinc-400">
+            {{ t('worktrees.start') }}
+            <select v-model="startSession" class="input">
+              <option :value="null">{{ t('worktrees.startNone') }}</option>
+              <option value="claude">claude</option>
+              <option value="bash">bash</option>
+            </select>
+          </label>
+          <label class="flex items-center gap-1 text-xs text-zinc-400"><input v-model="newBranch" type="checkbox" /> {{ t('worktrees.newBranch') }}</label>
+        </template>
+
+        <!-- mode branche principale : option git (branche actuelle ou nouvelle branche) + commande -->
+        <template v-else>
+          <label class="flex flex-col gap-1 text-xs text-zinc-400">
+            {{ t('worktrees.branchModeLabel') }}
+            <select v-model="mainBranchMode" class="input">
+              <option value="current">{{ t('worktrees.currentBranch') }}</option>
+              <option value="new">{{ t('worktrees.newBranchOpt') }}</option>
+            </select>
+          </label>
+          <label v-if="mainBranchMode === 'new'" class="flex flex-1 flex-col gap-1 text-xs text-zinc-400">
+            {{ t('worktrees.branch') }}
+            <input v-model="branch" class="input font-mono" placeholder="feature/…" />
+          </label>
+          <label class="flex flex-col gap-1 text-xs text-zinc-400">
+            {{ t('worktrees.commandLabel') }}
+            <select v-model="mainCommand" class="input">
+              <option value="claude">claude</option>
+              <option value="bash">bash</option>
+            </select>
+          </label>
+        </template>
+
+        <BaseButton type="submit" variant="primary" size="sm" :loading="busy" :disabled="!canSubmit">
+          {{ mode === 'worktree' ? t('worktrees.create') : t('worktrees.start') }}
+        </BaseButton>
+      </div>
     </form>
     <p v-if="error" class="text-sm text-red-400">{{ error }}</p>
 
@@ -66,20 +101,37 @@ const toasts = useToastsStore();
 // applique le tri/filtre/recherche partagé (toolbar du dashboard) aux worktrees de ce repo.
 const worktrees = computed(() => view.apply(store.worktreesForRepo(props.repo.id)));
 const creating = ref(false);
+const mode = ref<'worktree' | 'main'>('worktree');
 const branch = ref('');
 const newBranch = ref(true);
 const startSession = ref<'claude' | 'bash' | null>(null);
+const mainBranchMode = ref<'current' | 'new'>('current');
+const mainCommand = ref<'claude' | 'bash'>('claude');
 const busy = ref(false);
 const error = ref<string | null>(null);
 
+// worktree : branche obligatoire ; branche principale + nouvelle branche : branche obligatoire ; sinon OK.
+const canSubmit = computed(() =>
+  mode.value === 'worktree' ? branch.value.trim() !== '' : mainBranchMode.value === 'current' || branch.value.trim() !== '',
+);
+
 async function onCreate(): Promise<void> {
   busy.value = true;
   error.value = null;
   try {
-    await store.createWorktree(props.repo.id, { branch: branch.value.trim(), newBranch: newBranch.value, startSession: startSession.value });
+    if (mode.value === 'worktree') {
+      await store.createWorktree(props.repo.id, { branch: branch.value.trim(), newBranch: newBranch.value, startSession: startSession.value });
+      toasts.success(t('toast.worktreeCreated'));
+    } else {
+      // bosser sur la branche principale : session dans le checkout principal, avec création de branche optionnelle.
+      await store.startMainSession(props.repo.id, {
+        command: mainCommand.value,
+        ...(mainBranchMode.value === 'new' ? { branch: branch.value.trim(), newBranch: true } : {}),
+      });
+      toasts.success(t('toast.sessionCreated'));
+    }
     branch.value = '';
     creating.value = false;
-    toasts.success(t('toast.worktreeCreated'));
   } catch (err) {
     error.value = err instanceof Error ? err.message : String(err);
   } finally {